This invention relates to an automatic compensation circuit for automatically compensating a time constant of a filter, in particular, an automatic compensation circuit for automatically compensating a time constant of a filter used as an active filter or the like.
Conventionally, an integrated circuit constituted of inductor, capacitor, and the like, has been often used as a filter for an electronic circuit. In response to the request for downsizing of a circuit with the progress of the integration technique, an active filter has been developed in recent years such that a filter can be contained in an integrated circuit.
In many cases, the active filter contained in an IC is formed to have a variable transconductance in order to compensate the variance of qualities of the elements in the filter. The active filter is constituted of a voltage-current conversion section the gain of which is determined by an inner resistance value and a Gilbert cell section capable of changing a gain with use of a current ratio. By controlling the current ratio, the variance of resistance value and capacitance value of the resistive elements and capacitive elements in the filter can be compensated.
FIG. 1 is a block diagram showing the constitution of a compensation circuit for compensating the conventional active filter.
In this diagram, a filter 2 having a center frequency f.sub.0 is connected to a band pass filter (BPF) 4 having a variable time constant and a center frequency f.sub.0. The time constants of these filters can be changed by a control voltage Vc. The output f.sub.0 of the band pass filter 4 is fed back to an input terminal of the band pass filter 4 through an amplitude-limiting circuit 6, and then supplied to an APC circuit (phase detector circuit) 8.
The APC circuit 8 compares the input frequency signal f.sub.0 with a reference frequency f.sub.s with use of a feedback loop to generate a voltage signal corresponding to the phase difference therebetween. The generated voltage signal is supplied as the control voltage Vc to the band pass filter 4 and the filter 2.
The filter 2 and the band pass filter 4 are connected to a resistor R and capacitors C1 and C2, which are provided to supply the control voltage Vc to the band pass filter 4 and the filter 2. The APC circuit 8 is connected to a frequency discriminator 10.
With such a constitution, when the output signal of the band pass filter 4 is fed back by the amplitude-limiting circuit 6 such that the input signal and the output signal of the band pass filter 4 have the same phase, the band pass filter 4 functions as an oscillator having an oscillation frequency f.sub.o. When the oscillation frequency f.sub.o is fed back as the control voltage Vc to the band pass filter 4 (through the APC circuit), a feedback loop for setting the frequency as f.sub.o =f.sub.s is obtained.
In this time, if the filter 2 having the same constitution as that of the band pass filter 4 is also controlled with use of the control voltage Vc, a predetermined relationship can be obtained between the time constant of the filter 2 and the reference frequency signal f.sub.s, and the time constant is automatically compensated by the reference frequency signal f.sub.s.
As should be clear from the above, this circuit functions as a PLL (Phase-Locked Loop), and the band pass filter 4 and the amplitude-limitting circuit 6 function as a VCO (voltage-controlled oscillator). With this constitution, the frequency characteristics of the filter can be compensated by locking the phase with use of the PLL such that the cut-off frequency f.sub.o of the VCO is set to be equal to the reference frequency f.sub.s.
In such a kind of filter, the time constant is generally determined by a resistance value and a capacitance value. These elements for determining the time constant, however, cannot be formed to have sufficient precision in an IC. In order to improve the precision of the time constant, the currents or the like supplied to the elements are set to be variable.
However, the variance of the resistance and capacitance values is so large that the time constant compensation amount will also increase. In order to cover such a large compensation amount, the variable scope of the gain also must be set wide. The scope of the gain must be set to obtain the relationship f.sub.min &lt;f.sub.s &lt;f.sub.max so that the scope constantly covers a predetermined reference frequency f.sub.s however wide the resistance and capacitance values are varied. The resistance value is generally varied within an error of .+-.20% and the capacitance value is varied within an error of .+-.10% when a variable scope is set within .+-.30% with regard to the reference frequency f.sub.s as shown in FIG. 2. The oscillation frequency f.sub.o of the VCO thus must have a variable scope covering an error as large as .+-.30% or more, with allowance.
As described above, the variable scope of the gain is too wide, and thus the dynamic range for the control is made too small. In order to compensate the dynamic range, the conventional filter needs to be provided with a frequency discriminator 10. The frequency discriminator 10 sets the oscillation frequency f.sub.o of the VCO within a scope such that such a scope as f.sub.0 =f.sub.s .+-.10% is obtained, thereby the PLL locks the phase.
Further, such a PLL comprising the VCO as an oscillator needs to be provided with a frequency limitter, otherwise the PLL does not lock the phase.